1. Field of the Invention
The present invention relates to a fixing device for fixing a developed image of toner by heating and fusing the toner onto a sheet of paper or the like (recording sheet), and an image-forming apparatus having such a fixing device.
2. Description of Related Art
In general, the image-forming apparatus, such as a laser printer, includes a fixing device for fixing a developed image of toner, which has been transferred from a photoconductor to a sheet, on the sheet by heating and fusing the toner. The fixing device of a type known in the art, as disclosed in JP 6-308847 A for example, includes a heating roller configured to rotate on an axis fixed to a body of the fixing device, a driving force input element typically disposed at an end of the heating roller (and fixed coaxially to the heating roller and configured to rotate together with the heating roller), and a pressure roller configured to be pressed against the heating roller and to become deformed elastically when pressed against the heating roller.
In this type of the fixing device, typically, a driving force supplied from a driving source installed in the image-forming apparatus is transmitted by a plurality of gears to the driving force input element fixed to the heating roller, which causes the heating roller to rotate, and the pressure roller pressed against the heating roller is caused to rotate according as the heating roller rotates.
It has turned out desirable in recent years to be able to increase a nip width between the pressure roller and the heating roller (contact width) in the circumferential direction of the rollers (perpendicular to the axes), so as to increase a time of contact or an area of contact between the heating roller and the sheet, thereby making the heating roller operable at lower temperatures and/or at higher speeds. In the ‘heating roller driving’ type fixing device as described above, however, if the nip width were increased too much, the heating roller would possibly slip on the sheet along the deformed face of the pressure roller.
To be more specific, assuming that the pressing force of the pressure roller to the heating roller were increased or the contact surface of the pressure roller were made of a softer material in order to make the nip width greater, a portion of the pressure roller opposed to the heating roller would become recessed significantly when the pressure roller is pressed against the heating roller. Such a deformed (recessed) shape of the pressure roller (with its roundness impaired) however would possibly make its feeding resistance (or a resisting force exerted by the pressure roller on a sheet being fed forward) greater than a feeding force exerted by the heating roller on the sheet, and thus would disadvantageously cause a slip to occur between the heating roller and the sheet.
The inventors named in the present application, in an attempt to eliminate the above disadvantages, have discovered that the slip of a sheet as described above can be suppressed with the help of a particular arrangement (‘pressure roller driving’ scheme) where a driving force from the driving source is transmitted to a pressure roller, instead of a heating roller, to cause the pressure roller to rotate and the heating roller is in turn caused to rotate according as the pressure roller rotates.
This ‘pressure roller driving’ arrangement where the rotating pressure roller causes the heating roller to rotate typically includes: a heating roller incorporating a heat source that is fixed so as to rotate together with the heating roller; a pressure roller disposed in parallel with the heating roller; a pressing spring configured to press the heating roller against the pressure roller; and a driving force input element disposed at an end of the pressure roller (and fixed coaxially to the pressure roller and configured to rotate together with the pressure roller).
In a fixing device having an arrangement as described above, a driving force from a driving source disposed in an image-forming apparatus is transmitted by a plurality of gears to the driving force input element for the pressure roller, so that the pressure roller supplied with the driving force is caused to rotate while the heating roller is caused (driven) to rotate following the rotation of the pressure roller. The heating roller is configured to reciprocate toward and away from the pressure roller, in order to absorb the variations in an outside diameter.
In the ‘pressure roller driving’ scheme, however, it was unclear how much the nip width should be increased to make the heating roller practicably operable at lower temperatures. Moreover, since the ‘pressure roller driving’ scheme would inevitably cause the heat source to vibrate (reciprocate) together with the heating roller, it has been believed that the ‘heating roller driving’ scheme should be preferable to prevent damage to the heat source.
With this in view, it is appreciated that a problem in need of solution is to provide a fixing device and an image-forming apparatus with a heating roller operable at lower temperatures, which could be realized ideally by increasing a nip width but believed to be impossible due to a slip that would occur in the ‘heating roller driving’ scheme.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.